Photographs from each of these examples can be viewed at the bottom of this page.
Riverbank revetment at Great Meadow
A section of bank above the weir at Great Meadow on the Silverburn River had previously been protected using stobs and mesh back-filled with stone. Failure of the revetment had left a sheer, rapidly eroding bank and the potential for the stream eventually to migrate such as to bypass the weir.
In September 2006, employing a design produced by Salix River and Wetland Services Ltd., the then Land Drainage Section of the Department of Transport (now the Flood Risk Management Team, Manx Utilities Authority) repaired the bank using 'soft'-engineering techniques, which provide a more aesthetically pleasing and wildlife-friendly result. Such methods can also prove very durable and cost-effective. The use of faggots helps to absorb energy from the water flow, while the geotextile matting preserves the integrity of the bank as vegetation re-establishes.
After passing the test of a very wet winter, a selection of Manx native trees and shrubs were planted along the site in February 2007 to further increase the bank's stability and wildlife value.
Wildlife potential was further enhanced by the incorporation of a small backwater, seeded with a mixture of native marginal plants.
Coir roll is usually installed pre-established with marginal plants, which themselves act as an anti-erosion barrier. However, imported pre-established rolls would have entailed introducing plants not of Manx genotype so, for this repair, Manx native marginals raised by the Wildflowers of Mann project were planted behind the rolls in early spring.
An area has now been set up at Mullen-e-Cloie by Andree Dubbeldam of Wildflowers of Mann to pre-establish coir rolls ready for similar techniques to be employed by the Manx Utilities Authority elsewhere on the Island.
The faggots used at Great Meadow were sourced from the UK. A more sustainable supply was subsequently identified in the form of willow faggots produced by the Manx Wildlife Trust. The Trust’s management of the Close Sartfield Nature Reserve had, for some time, involved a willow coppicing regime but, as there had previously been little use for the cut material, most of it was burned on site. The extra funds received by the Manx Wildlife Trust as a result of this initiative will further contribute to conservation on the Island.
Log and 'Christmas tree' revetment
In September 2011, Forestry staff and the River Management Project Officer installed a short section of Log and 'Christmas tree' revetment, a technique used extensively in Ireland, on the right bank of the Silverburn downstream of Silverdale Cafe. A double row of lodgepole pine logs protects the toe of the bank while conifer tops attached to the logs dissipate the energy of the flow and collect silt during high flows. The work was able to be done by hand thereby minimising costs and disturbance to the riverbed.
The lower reaches of several of the Island's rivers have been straightened in the past, which can reduce the diversity of in-stream habitat to the detriment of fish populations. For instance, such channels tend to be dominated by shallow riffle and glide sequences, and contain fewer pools than one would find on a naturally meandering river.
Simple enhancement techniques, such as the installation of a series of boulders on a 30m stretch of the Silverburn in autumn 2006 can sometimes enhance such areas. Flow is diversified, the number of available fish territories may be increased and spawning gravels collect downstream of each boulder. Fish surveys indicate that the site has since supported a greater density of large trout.
In October 2009, a channel constrictor was installed in the River Neb near Ballawyllin to create and maintain a new pool. Pools provide fish with refuges from drought conditions, are the main habitat for large trout, provide safe resting places for migratory salmonids, and the deep, well-oxygenated deposits of gravel which tend to form at the tail-end of pools also provide some of the most productive spawning habitat. The constrictor consists of a pair of triangular structures made of faggots and coir roll, which narrow the channel by two thirds at that point but flood over at high flows. Post-construction increases in trout and salmon fry immediately above and below the site may have resulted from the pool attracting spawning salmon and sea trout to the area.
If you own a watercourse, which you think may benefit from enhancement, contact:
+44 1624 651544
Management of riparian trees
Overhanging vegetation provides valuable cover for fish. However, extensive stretches of heavy shade can limit salmon and trout populations by reducing the growth of algae on which many of their invertebrate prey feed. The reduced light can also limit the growth of herbaceous plants, which keep the banks stable. During winter 2007/2008, students and staff from Ramsey Grammar School helped the River Management Officer to coppice and trim bankside trees along parts of the Santonburn and River Neb.
This weir on the Santon Burn has been a major obstacle to migratory fish for over 200 years. Fish surveys and anecdotal information gathered during a farm watercourse survey in the catchment indicated that, in recent years, it had become almost completely impassable due to changes in the nature of the stream bed. Following consultation with the landowners and the former Department of Transport (DoT), a proposal was submitted by the River Management Officer for the Works Division of DoT to build a bypass channel utilising the disused mill race adjacent to the weir.
Sea trout began to use the pass soon after its construction in March 2009, since which juvenile trout numbers have increased substantially in the upper reaches of the river. However, wild salmon have so far appeared reluctant to take the opportunity to access the excellent habitat available above, possibly due to their tendency to return to the particular area of river in which they hatched. Hatchery reared salmon, bred from adults collected downstream of the pass and introduced to the river's upper reaches, appear to be doing well and releases are planned to continue until surveys indicate that natural recruitment is taking place.